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Evolution of One Solar Mass Star

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Evolution of One Solar Mass Star

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    1. Evolution of One Solar Mass Star

    2. Stages 4-7: Protostar

    3. Protostar Source of luminosity is gravitational contraction ( primarily of the core ) Core is always getting smaller and hotter

    4. Radiative Protostar When heat transfer is dominated by radiation – core heating results in higher temperatures at the photosphere The effect of the star getting smaller cancels the effect of higher surface temperature, so the luminosity remains constant Evolutionary track is vertically down

    5. Convective Protostar When heat transfer is dominated by convection – core heating does not result in higher temperatures at the photosphere Surface temperature stays constant and luminosity goes down because the star as a whole is getting smaller. Evolutionary track is vertically down

    6. Stage 7: Main Sequence Stable Hydrogen to Helium fusion in the core Lasts 10 Billion years Heat transport mainly radiation but some convection in outer layers

    7. Stages 7 – 9: Red Giant

    8. After the Main sequence Stages 7 to 8 Hydrogen in Core exhausted Core contracts and heats, igniting Hydrogen fusion in a shell just outside the core High opacity envelope expands and cools Luminosity stays constant because the effects of the surface getting hotter ( hence brightening ) cancels the effect the star having more surface area because it is expanding

    9. The Red Giant Branch Stages 8 – 9 : 8 - Core becomes degenerate, very small and hot This increases the heat transfer to the fusion shell, heating it up and driving up the rate of fusion and hence the luminosity High opacity envelope expands but doesn’t cool due to the increase in luminosity Some mass loss due to escaping gas from the envelope, since the outer parts are now far removed from the gravitational attraction of the core

    10. Stage 9 : Helium Flash

    11. Helium Flash In the degenerate state heat transfer is dominated by conduction and is extremely rapid , leading to explosively increasing nuclear energy generation rates. The energy generation gives rise to very high radiation pressure which basically lifts the core out of its degenerate state ( this is the only time in the evolution of a star that the size of its core actually increases ) .

    12. Stages 9 – 10 : Horizontal Branch

    13. Cepheid Variables Cepheids pulse because of what amounts to a runaway "safety valve" in which the star's luminosity oscillates around an equilibrium value The "safety valve" of a non-degenerate core involves an increase in temperature leading to an increase in radiation pressure which leads to expansion which leads to cooling. In degenerate stars there is no "safety valve " because increase in radiation pressure has almost no effect. It is the degeneracy pressure that is controlling the size of the core, it just keeps on getting hotter

    14. Stages 10 – 11 : Asymptotic Giant Branch

    15. Stages 11 – 12 : Planetary Nebula

    16. Stages 12 – 13 : Descent to White Dwarf

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